In case of USE_POSTFIX, the POSTFIX is dependent
on the CMAKE_BUILD_TYPE.
Use the correct POSTFIX also in the generated pugixml.pc file.

This results in the following contents of pugixml.pc:
- Release:
  Libs: -L${libdir} -lpugixml
- RelWithDebInfo
  Libs: -L${libdir} -lpugixml_r
- MinSizeRel:
  Libs: -L${libdir} -lpugixml_m
- Debug:
  Libs: -L${libdir} -lpugixml_d